The present invention relates to an apparatus and method for in-situ thickness and stoichiometry measurement of thin films made by, for example, molecular beam epitaxy, chemical vapor deposition, sputtering, plasma coating, etc.
The present high demand for electronic and optoelectronic devices, such as quantum well laser diodes (see, Kasukawa, N. Yokouchi, N. Yamanaka, N. Iwai and T. Matsuda, Jpn. J. Appl. Phys. 34, L965, 1995) or optical modulators (see, R. Y.-F Yip, A. Axc3xaft-Ouali, A. Bensaada, P. Desjardins, M. Beaudoin, L. Isnard, J. L. Brebner, J. F. Currie and R. A. Masut, J. Appl. Phys. 81, 1905 (1997) and references therein, have pushed the epitaxial requirements to precisions and reproducibility in alloy compositions and thickness to the monolayer level. One method for measuring growth rate in Molecular Beam Epitaxy (MBE) is Reflection High Energy Electron Diffraction (RHEED). This method relies on the intensity oscillations of the diffracted electron beam caused by variable coverage during a layer by layer growth. RHEED measures growth rate only at the beginning of growth as the oscillations die out quickly in practice. Moreover, this method is not applicable when growth proceeds by other mechanisms, such as step-flow mode, and is not useful for measurements with sample rotation. These restrictions limit the usefulness of RHEED for real time feedback control of MBE growth.
These considerations have led to developments in ellipsometry where the thickness of grown films is inferred from the changes in the optical reflectivity and thin film interference effects (see, C. H. Kuo, M. D. Boonzaayer, M. F. DeHerrera, D. K. Schroder, G. N. Maracas and B. Johs, J. Cryst, Growth, (to be published, 1997) and references therein. Kuo, for instance, have achieved in-situ thickness control of AlAs/GaAs Distributed Bragg Reflector (DBR) structures with a 0.3% reproducibility. The method relies on dynamic model fitting of the optical spectra during growth, which is sensitive to the optical properties of the growing materials. These depend on both the thickness and composition of the material, and neglects the effects of scattering due to surface roughness (see, M. K. Nissen, C. Lavoie, S. Eisebitt, T. Pinnington and T. Tiedje, Scanning Microscopy, 8, 936 (1994); Lavoie, T. Pinnington, E. Nodwell, T. Tiedje, R. S. Goldman, K. L. Kavanagh and J. L. Hutter, Appl. Phys. Lett., 67,3744 (1995)). To install the system on an MBE chamber, one optical port is used for the light source and another port for the ellipsometer. To allow for ellipsometric measurements under sample rotation conditions, it is necessary that the substrate surface remain parallel to the sample rotation. To achieve this, Kuo et al. developed a wobble free substrate rotation mechanism (see, C. H. Kuo, M. D. Boonzaayer, M. F. DeHerrera, D. K. Schroder, G. N. Maracas and B. Johs, J. Cryst, Growth, (to be published, 1997) and references therein).
The present inventor has taken a different approach to in-situ thickness and growth rate measurements in MBE with the use of Alpha Particle Energy Loss Method (AEL) (see, Kelson, Y. Levy and E. Redmard, J. Phys. D: Appl. Phys. 28, 100 (1995); Kelson, Y. Levy, D. Racah, E. Redmard, M. Beaudoin, T. Pinnington, T. Tiedje and U. Giesen, J. Phys. D: Appl. Phys. D: Appl. Phys. 30, 131 (1997). This approach is easier to implement than ellipsometry while providing absolute thickness measurements that do not depend on optical properties. The AEL method has already been implemented by the inventor of the present invention off-line, with good results, for thickness and stochiometry measurements of several materials including semiconductors (see, Kelson, Y. Levy and E. Redmard, J. Phys. D: Appl. Phys. 28, 100 (1995). It is an object of the present invention to provide a method and apparatus for on line, in situ implementation of the AEL method for the thickness and stoichiometry of thin films, particularly film made by Molecular Beam Epitaxy. It is a more particular object of the present invention to provide on line, in-situ implementation of the AEL method for the growth of III-V semiconductors, and other films with MBE.
In accordance with the above objects, the present invention provides an apparatus for measuring the thickness of films grown on a substrate in a growth chamber. The apparatus comprises: a protective housing having an aperture opening into the growth chamber, a solid state detector disposed in the protective housing, a shutter for opening and closing the aperture, a shield disposed in the housing between the aperture and the solid state detector for shielding the detector, and a calibration source disposed between the shield and the detector for calibrating the measurements made by the detector.
The apparatus also preferably comprises a second calibration source disposed between the shutter and the shield, for measuring deposition on the shield. A second shutter is provided for selectively exposing and containing the second calibration source.
A temperature detector is preferably positioned to detect the temperature of the protective housing and a cooling device is constructed to cool the housing.
In one preferred embodiment, a device for focusing said detector on one or more predetermined regions of the substrate is additionally provided.
Also in accordance with the above objects, the present invention provides a method for measuring a property of a film deposited on a substrate in a growth chamber comprising the steps of: implanting one or more alpha sources in a substrate, placing the substrate in a growth chamber, growing a film on the substrate, detecting the loss of energy of alpha emissions of the substrate as a film is grown on the substrate in the chamber with a detector, protecting the detector from the growth chamber with a shield, calibrating the detector with a calibration source disposed between the detector and the shield, and calculating the property of the film on the substrate using the loss of energy of the alpha emissions from the source.
The method preferably further comprises the step of calibrating the detector with a second calibration source to compensate for deposition on said shield from the growth chamber. The property measured is thickness of the film, stoichiometry of the film, or a combination of both.
In one embodiment, the step of detecting comprises detecting the loss of energy of two or more alpha lines. In a still further embodiment, the step of implantation comprises recoil implantation using an implantation source made by irradiation with a beam of 228Fr. In yet another embodiment, the step of implantation comprises recoil implantation using an implantation source made by irradiation with a beam of 227Fr.
The xcex1-particle source is preferably selected from 228Th, or 227Th.
In another embodiment, the method further comprises the step of focusing the detector on one or more predetermined regions of the substrate. In this embodiment one or more sources can be implanted in a predetermined pattern on the substrate.
Further objects, features and advantages of the present invention will become apparent from the Detailed Description of the Preferred Embodiments, when considered together with the attached drawings.